libcamera: process: Remove ProcessManager singleton

The `ProcessManager` is a singleton class to handle `SIGCHLD` signals
and report the exit status to the particular `Process` instance.

However, having a singleton in a library is not favourable and it is
even less favourable if it installs a signal handler.

Using pidfd it is possible to avoid the need for the signal handler;
and the `Process` objects can watch their pidfd themselves, eliminating
the need for the `ProcessManager` class altogether.

`P_PIDFD` for `waitid()` was introduced in Linux 5.4, so this change
raises the minimum supported kernel version. `clone3()`, `CLONE_PIDFD`,
`pidfd_send_signal()` were all introduced earlier.

Furthermore, the call to the `unshare()` system call can be removed
as those options can be passed to `clone3()` directly.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/camera_manager.h

@@ -65,7 +65,6 @@ private:
 	std::unique_ptr<DeviceEnumerator> enumerator_;
 
 	std::unique_ptr<IPAManager> ipaManager_;
-	ProcessManager processManager_;
 };
 
 } /* namespace libcamera */

include/libcamera/internal/process.h

@@ -7,7 +7,6 @@
 
 #pragma once
 
-#include <signal.h>
 #include <string>
 
 #include <libcamera/base/class.h>
@@ -45,41 +44,14 @@ public:
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(Process)
 
-	int isolate();
-	void died(int wstatus);
+	void onPidfdNotify();
 
 	pid_t pid_;
 	enum ExitStatus exitStatus_;
 	int exitCode_;
 
-	friend class ProcessManager;
-};
-
-class ProcessManager
-{
-public:
-	ProcessManager();
-	~ProcessManager();
-
-	void registerProcess(Process *proc);
-
-	static ProcessManager *instance();
-
-	int writePipe() const;
-
-	const struct sigaction &oldsa() const;
-
-private:
-	static ProcessManager *self_;
-
-	void sighandler();
-
-	std::list<Process *> processes_;
-
-	struct sigaction oldsa_;
-
-	EventNotifier *sigEvent_;
-	UniqueFD pipe_[2];
+	UniqueFD pidfd_;
+	std::unique_ptr<EventNotifier> pidfdNotify_;
 };
 
 } /* namespace libcamera */